Mississippi Irrigation Systems

35% of exam (14 questions) — Highest priority topic

Key Concepts

PVC Pipe Classification

• Schedule 40: Rated by wall thickness; higher pressure rating for smaller diameters. General-purpose use.
• Class 160: Rated for 160 psi working pressure (SDR 26). Common for mainlines.
• Class 200: Rated for 200 psi working pressure (SDR 21). Higher-pressure applications.
• Class 315: Rated for 315 psi working pressure. Heaviest-duty lateral and mainline use.
• SDR (Standard Dimension Ratio) = outside diameter ÷ wall thickness. Lower SDR = thicker wall = higher pressure rating.

Water Velocity Limits

• Maximum velocity: 5 ft/sec for lateral lines.
• Maximum velocity: 7 ft/sec for mainlines (some references allow up to 7.5 ft/sec).
• Exceeding velocity limits causes pipe noise, erosion of fittings, and dramatically increases surge pressure risk.
• Velocity is the primary driver of pipe sizing — if velocity is too high, increase pipe diameter.

Friction Loss

• Friction loss increases as water travels through pipe due to internal pipe roughness slowing water molecules.
• Longer pipe runs = greater accumulated pressure loss.
• Pressure changes 0.433 psi per 1 ft of elevation change (rise = pressure loss; fall = pressure gain).
• Use the H-pattern (balanced circuit) to equalize pressure at all heads — route the mainline to intersect near the center of each circuit's heads.

Key Concepts

Hydrozoning (Site Division) Divide sites into separate irrigation zones based on three factors: plant water needs (evapotranspiration rates vary by plant type and season), soil type (affects infiltration and runoff), and slope (affects runoff and distribution uniformity). Never mix high- and low-water-use plants on the same zone.

Sprinkler Head Selection & Placement - Spray heads: radii 1–16 ft; high precipitation rate; best for smaller areas - Rotary heads: throw 1–108 ft (some agricultural heads to 650 ft); lower precipitation rate; more cost-efficient per square foot - Rotary heads must run up to 5× longer than spray heads to deliver the same water volume - Select head type based on area size, available pressure, and water window (time available to irrigate)

Head-to-Head Coverage Manufacturers recommend spacing heads at 50% of diameter (= 100% of radius). This is the definition of head-to-head coverage. Why: a single head only sustains good plant growth within the inner two-thirds of its radius. The outer one-third does not receive adequate water without overlap from adjacent heads.

Square vs. Triangular Spacing - Square spacing: Heads placed at corners of a square grid. Spacing = radius (R). Less efficient coverage per head. - Triangular (staggered) spacing: Heads offset in alternating rows. Achieves approximately 15% better distribution efficiency than square spacing. Preferred when site geometry allows.

Key Concepts

Types of Sprinkler Heads - Fixed spray heads (e.g., Series 570Z): Short-range, static pattern, used for small turf and ground cover up to 16 ft. Higher precipitation rate, lower flow requirement. - Rotary heads (e.g., Series 300 Stream Rotors, XP-300, Super 600/700): Rotating stream, used for medium-to-large turf areas (28 ft–52 ft+). Lower precipitation rate, better for clay soils. - Large rotary heads (640, 2001 series): Parks, athletic fields. Golf-specific heads: 650–780 series.

Spray Patterns - Rectilinear patterns: Fixed rectangular coverage (e.g., 3×12 ft, 9×18 ft). Best for medians, narrow strips between sidewalk and curb. - Circular patterns: Defined by (1) radius of throw, (2) arc of pattern, (3) trajectory angle. Available from 3 ft to 100+ ft radius. - Common arc configurations: quarter-circle (90°), half-circle (180°), three-quarter (270°), full circle (360°). Many heads have adjustable arcs (0°–360°).

Trajectory Angles - 0°: Flat spray — under low shrubs, tops of hills. - 10°: Low angle — windy conditions, shrubs, ground cover, turf. - 25°–28°: Standard — most turf applications. - 35°: Stream spray — ground cover, shrubs, turf.

Head-to-Head Coverage - A single sprinkler provides adequate water for only two-thirds of its throw radius. The outer one-third is insufficient for sustained plant growth. - To compensate, overlap patterns so heads are spaced at 50% of the diameter (= 100% of the radius). This is the industry-standard "head-to-head coverage."

Key Concepts

Controller Types - Electromechanical: Uses mechanical clock/timer components; reliable but limited programming flexibility. - Solid-state: Microprocessor-based; more programming options, no moving parts. - Hybrid: Combines mechanical and solid-state components.

Controller Programming Terms - Station: One output channel controlling one valve (or group of valves wired together). - Program: A complete schedule including start times and station run times. - Start time: The time of day a program begins running. - Watering cycle: The sequence in which stations run after a start time is triggered.

Electrical Fundamentals (Analogies to Water) - Volt (V): Electrical pressure — analogous to psi. Controllers step down 120V to 24V via transformer. - Amp (A): Flow of electrons — analogous to gpm. Volts × Amps = Watts. - Watt (W): Unit of power (e.g., 120V × 10A = 1,200W). - Voltage drop: Loss of voltage over wire length — analogous to friction loss in pipe. Insufficient voltage prevents solenoids from opening or staying open. - Inrush current: Higher amperage needed to open a solenoid valve. - Holding current: Lower amperage needed to keep the valve open.

Key Concepts

Trench Excavation Requirements

• Trench width should be sufficient to allow proper pipe placement and compaction of backfill on both sides of the pipe
• Trench bottom must be graded smooth and free of rocks, sharp objects, or debris that could damage pipe
• Grade the trench bottom continuously toward a drainage outlet to prevent water pooling (critical for freeze protection)
• Pipe grading for drainage/freeze protection: Lateral lines and mainlines must slope to allow complete drainage; dead-end lines require drain valves or manual drain points

Minimum Cover Depths

• Mainlines: Minimum 12 inches of cover in non-traffic areas; deeper cover required under paved or trafficked surfaces
• Lateral lines: Minimum 6–8 inches in non-traffic turf areas
• Sleeved crossings under paved areas: Sleeve must extend beyond the edge of pavement on both sides; minimum cover depth increases under pavement — typically 18–24 inches depending on traffic load
• Sleeve diameter should be at least twice the diameter of the pipe passing through it

Key Concepts

Types of Irrigation Pumps - Horizontal centrifugal pumps: Most common for surface water sources (lakes, ponds). Motor and pump shaft are horizontal. Best when water source is at or near surface level. - Vertical turbine pumps: Used for deep wells or sumps where the water source is below grade. Impeller is submerged; motor sits above ground on a vertical shaft.

How Centrifugal Pumps Work - The pump consists of an impeller (rotating blades) inside a volute (spiral-shaped housing/case). - A motor rotates the impeller via a shaft, creating a low-pressure (vacuum) zone at the intake. - Atmospheric pressure (14.7 psi at sea level) pushes water into the low-pressure zone; the impeller flings water outward, converting velocity to pressure in the volute. - Suction lift is limited by atmospheric pressure — practical maximum is approximately 25 ft at sea level.

Multistage Pumps (Stacked Impellers) - Multiple impellers installed in series within one pump housing. - Each impeller stage adds additional pressure (head). - Used when a single impeller cannot generate sufficient pressure for the system requirement.